Matthew Dalton

PARIS—When an EgyptAir flight crashed on its way from Paris to Cairo in May 2016, international law designated the Egyptian government to lead a probe to find out why the jet plunged suddenly into the Mediterranean Sea, killing all 66 people aboard.

That investigation was never completed. Instead, Egyptian authorities said a bomb had likely brought down the plane and withheld key evidence from French investigators, citing the secrecy of their counterterrorism inquiry.